The present invention relates to a oxygen separation and combustion apparatus and method that can be applied to such devices as a boiler or a nitrogen generator in which oxygen separated from an oxygen containing gas by oxygen transport membranes supports combustion of a fuel within a combustion chamber and temperature of the oxygen transport membranes is controlled by fluid circulating within fluid passages passing through the combustion chamber.
Growing concerns about environmental issues, such as global warming and pollutant emissions, are driving industries to explore new ways to increase efficiency and reduce emissions of pollutants. This is particularly true for fossil fuel fired combustion systems, which represent one of the largest sources of carbon dioxide and air pollution emissions. One effective way to reduce emissions and to increase efficiency is to use oxygen, or oxygen enriched air, in the combustion process. The use of oxygen or oxygen enriched air reduces stack heat losses, which increases the system efficiency, while at the same time reducing NOx emissions. Further, the concentration of carbon dioxide in the flue gas is higher since there is little or no nitrogen to act as a diluent. The higher carbon dioxide concentration enhances carbon dioxide recovery options. Oxygen use in the prior art has been limited to those processes with high exhaust temperatures, such as glass furnaces. In such applications, the fuel savings and the benefits achieved are greater than the cost of the oxygen. In low exhaust temperature systems, such as boilers, the reverse is true. In these systems, the cost of oxygen produced with current technologies is more expensive than the available fuel savings. This makes oxygen use in such systems economically unattractive. Moreover, when the energy required to produce the oxygen is taken into consideration, the overall thermal efficiency decreases.
Oxygen transport membranes have been advantageously utilized in the prior art to produce oxygen for heat consuming oxygen separation and combustion apparatus and processes in a manner that results in a savings of energy that would otherwise have to be expended in the separation of oxygen. Oxygen transport membranes are fabricated from oxygen-selective, ion transport ceramics in the form of tubes or plates that are in themselves impervious to the flow of oxygen and other gases. Such ceramics, however, exhibit infinite oxygen selectivity at high temperatures by transporting oxygen ions through the membrane. In oxygen transport membranes, the oxygen is ionized on one surface of the membrane to form oxygen ions that are transported through the membrane. The oxygen ions on the opposite side of the membrane recombine to form oxygen with the production of electrons. Depending upon the type of ceramic, oxygen ions either flow through the membrane to ionize the oxygen or along separate electrical pathways within the membrane, or by an applied electric potential. Such solid electrolyte membranes are made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides having fluoride or perovskite structures.
In U.S. Pat. No. 5,888,272 oxygen transport membranes are integrated into a combustion process itself, with all the oxygen produced going directly into the combustor. The heated flue gases can then be routed to a process wherein the thermal energy can be used to heat a fluid or perform useful work. In one embodiment, flue gases are recycled through a bank of oxygen transport membrane tubes and enriched with oxygen. Typically the flue gas enters the bank containing anywhere from 1 to about 3 percent oxygen and leaves the bank containing from about 10 to about 30 percent oxygen by volume. The enriched flue gas is then sent to a combustion space where it is used to burn fuel. In another embodiment, called reactive purge, the oxygen transport membrane tubes are placed directly in the combustion space. A fuel diluted with flue gas is passed through the tubes and combust with the oxygen as it passes through the tubes. Thus oxygen production and combustion take place simultaneously.
As will be discussed, the present invention utilizes oxygen transport membranes to produce oxygen to support combustion within a oxygen separation and combustion apparatus such as a boiler in a manner that inherently reduces the energy expenditures involved in compressing an incoming oxygen containing feed to the membranes. The advantages of the present invention will become apparent from the following discussion.
In one aspect, the present invention provides an oxygen separation and combustion apparatus comprising a plurality of parallel oxygen transport membranes located within a combustion chamber. The plurality of parallel oxygen transport membranes serve to separate oxygen from an oxygen containing gas, thereby to provide the oxygen within the combustion chamber to support combustion of a fuel and generate heat. A plurality of fluid passages pass through the combustion chamber and are positioned so that a portion of the heat of combustion is transferred from the combustion to the oxygen transport membranes to heat the oxygen transport membranes to an operational temperature and a further portion of the heat is transferred from the combustion to the fluid passages to provide heat to heat the fluid and to promote stabilization of the operational temperature of the oxygen transport membranes. At least one inlet is provided for introducing at least the fuel into the combustion chamber and an exhaust from the combustion chamber discharges combustion products arising from combustion of the fuel. The exhaust and the at least one inlet are spaced apart from one another so that the combustion products flow in a direction predominantly parallel to the oxygen transport membranes.
The oxygen transport membranes and the fluid passages can be of tubular configuration. The direction of flow of the combustion products can either be countercurrent or co-current to gas flow of the oxygen containing gas within the oxygen transport membranes. Preferably, the oxygen transport membranes are closed at one end and open at the end to discharge an oxygen-depleted retentate and a plurality of coaxial lance tubes project into open ends of the oxygen transport membranes to supply the oxygen containing gas thereto. The at least one inlet can comprise an inlet to the combustion chamber for introducing a mixture of the fuel and a flue gas, if flue gas is required, into the combustion chamber. Alternatively, in case of open ended, tubular oxygen transport membrane units, the at least one inlet can comprise fuel nozzles located adjacent to the open ends of the oxygen transport membranes.
The fluid can be water and thus, the fluid heater can be a boiler. In such case, the fluid passages are interspersed between the oxygen transport membranes and the fluid passages and the oxygen transport membranes are parallel to one another. Preferably, the fluid passages communicate between fluid inlet and outlet manifolds to supply the fluid to the fluid passages and to discharge steam therefrom, respectively. In such case, the oxygen transport membranes project, from the open end thereof, from a retentate outlet manifold to discharge oxygen depleted air and the lance tubes project from an air inlet manifold.
In another aspect, the present invention provides an oxygen separation and combustion method in which an oxygen containing gas is introduced into a plurality of parallel oxygen transport membranes located within a combustion chamber. Oxygen is separated from the oxygen containing gas within the plurality of parallel oxygen transport membranes, thereby to provide oxygen within the combustion chamber. A fuel is introduced into the combustion chamber and the fuel is combusted within the combustion chamber in the presence of the oxygen to generate heat. The fluid is passed through a plurality of fluid passages also located within the combustion chamber and combustion products are discharged from the combustion chamber. The combustion products are discharged from the combustion chamber and the fuel is introduced so that the combustion products flow in a direction predominantly parallel to the oxygen transport membranes to provide a reactive purge to promote the separation of the oxygen from the oxygen containing gas. The fluid passages are positioned so that a portion of the heat is transferred from the combustion to the oxygen transport membranes to heat the oxygen transport membranes to an operational temperature and a further portion of the heat is transferred from the combustion to the fluid passages to provide heat to heat the fluid and to promote stabilization of the operational temperature of the oxygen transport membranes. The fluid can be water that is heated.
In either of the foregoing aspects of the present invention, and as used herein and in the claims, the term xe2x80x9cheatedxe2x80x9d means transferring heat to the fluid and thereby raising its temperature. Further, the term, xe2x80x9cwaterxe2x80x9d encompasses both water in liquid form and steam or a two-phase mixture of water and steam. Thus, as used herein and in the claims, the term, xe2x80x9cheatedxe2x80x9d when used in connection with water means raising the temperature of the water by any amount. As such, the temperature rise of water may or may not be sufficient to raise steam and if the water enters the heat transfer passages as steam, the steam will become superheated.
The integration of the oxygen transport membranes and the combustion system described above dramatically reduces the power requirement for oxygen production. The oxygen flux through an oxygen transport membrane is approximately proportional to the log of the partial pressure ratio between the source side and the product side, if mass transfer rate is controlled by the membrane itself. For example, to produce pure oxygen at one atmosphere absolute, the air must be compressed to about fifteen atmospheres. This results in a net energy requirement of approximately 160 kW/ton assuming expansion of the oxygen depleted air. Although this power requirement is less than conventional equipment, which is closer to 200 KW/ton, integrated the oxygen transport membrane unit with the combustion system of the boiler reduces this energy requirement still further by providing a reactive purge to consume the oxygen as it passes through the membrane. Such oxygen consumption produces an oxygen concentration on the product side of the oxygen transport membrane that is therefore always near zero. This provides such a large driving force that requires only minimal compression, typically just enough to move the air through the oxygen transport membrane. This can be accomplished with a blower instead of a more expensive compressor.
In a tubular membrane, since the largest driving force for oxygen separation occurs at the entry point of the air or other oxygen containing gas to the membrane, a countercurrent flow of combustion products provides a more fuel-rich and therefore oxygen lean conditions at the opposite end of the membrane, where less driving force is present, to further enhance the effect of the reactive purge.
Since fuel, flue gas and combustion products exist as a mixture within the combustion chamber, the fuel is diluted so that the driving force of the diffusion of the fuel to the surface of the oxygen transport membrane is reduced. At the same time, the oxygen flux through the membrane is low enough that, by in large, fuel rich conditions are encountered. Therefore, combustion of the fuel can be said to take place at or near the surface of the membrane. This of course depends on the degree of dilution.
The result of the location of the combustion in apparatus and methods in accordance with the present invention produces a heat of combustion that can cause a thermal runaway of the oxygen transport membrane resulting in damage and premature failure. In the present invention, the heat transfer passages, which can be interspersed steam tubes, act to withdraw the heat and thereby help stabilize the operational temperature of the oxygen transport membranes.
A further advantage that may be obtained from the present invention is a potential for a high degree of integration. Since the oxygen is produced at the point of use, no oxygen-safe piping is required. Further the energy required to heat the air and the fuel-flue gas mixture to the optimal operation temperature of the oxygen transport membrane comes directly from the oxygen transport membrane without concomitant heat loses that would otherwise occur with external piping. The integration also minimized the boiler/heater size and complexity. Since oxygen is produced in the unit, no other space is required for an on-site conventional air separation system. The location of the oxygen transport membranes and heat transfer passages within a combustion chamber also helps to minimize the overall footprint of a fluid heater of the present invention.
Another major benefit that may be obtained from the present invention is that high purity nitrogen can be produced as a byproduct. The high driving forces for oxygen transport allow for the production of such nitrogen with little or no oxygen. Furthermore, a fluid heater in accordance with the present invention will produce very little NOx since combustion takes place in the presence of oxygen instead of air. Since the oxygen is gradually added to the fuel-flue gas mixture as it passes through the combustion chamber, the combustion takes place under fuel rich conditions. Hence, the combustion is inherently staged with a long residence time in the fuel-rich regime, and with slow transition from fuel rich to fuel lean combustion to also lessen the possibility of NOx formation.